I hear you, you are not alone, lots of people have this exact same experience. It's really, really common, I was terrified as a child.
A couple of times I cried so much in the chair that the dentist told my parents to take me away again. Win. 😀
So, as an adult, what did I do so I could have my teeth treated ?
Absolutely, check in advance that the dentist you are going to see is used to dealing with people who are not 'comfortable' in the dentists chair.
There are two things that absolutely helped me. The first is to focus on breathing out.
It's not comfortable when someone is poking round your mouth, very easy to find you can't breath out. This makes all the tension and fear even worse. If you have to ask them to stop for a moment so you can breathe out.
Keep practising breathing out while they get on with things.
The second point is to have some control. My dentist has agreed a signal: If I raise my right hand he will immediately stop.
I've been going for decades now, had some significant work done and never had to actually stop him. I'm thinking breathe out, oh, that was a bit of a gasp but there I've managed to breath out. He is still working in my mouth. Does it hurt? No. Do I need him to stop? No, not just now. Ok breathe out again.
Then he says ok, rinse and spit and I am all done.
I hope you find someone sympathetic who can help you, too. It's such a relief when you are no longer haunted by this fear. Modern dentistry really is a miracle for me.